Nonaqueous-electrolyte batteries including lithium secondary batteries are being put to practical use in extensive applications ranging from power sources for appliances for so-called public use, such as portable telephones and notebook type personal computers, to vehicle-mounted power sources for driving motor vehicles or the like. However, nonaqueous-electrolyte batteries are increasingly required to have higher performance in recent years, and are required to attain battery characteristics, such as, for example, high capacity, high output, high-temperature storability, cycle characteristics, and high safety, on a high level.
In nonaqueous-electrolyte batteries, LiCoO2 is generally used in the positive electrodes and a carbon material capable of occluding and releasing lithium is generally used in the negative electrodes. As the nonaqueous electrolytic solutions, use is being made of electrolytic solutions prepared by dissolving an electrolyte salt represented by LiPF6 in a nonaqueous organic solvent such as ethylene carbonate or ethyl methyl carbonate.
Lithium cobalt oxide (LiCoO2), which is used as a positive-electrode active material as shown above, has a drawback that this substance in a charged state has low thermal stability and reduces battery safety. Extensive substances have hence been investigated in search of a positive-electrode active material usable as a substitute for LiCoO2.
As one class of substances among these, lithium-containing metal oxides having an olivine structure have recently received attention. For example, nonaqueous-electrolyte batteries employing LiFePO4 as a positive-electrode active material can be made to have improved cycle characteristics and improved battery safety by taking advantage of the high thermal and chemical stability of LiFePO4. In the case of applications such as, for example, hybrid vehicles, such properties are exceedingly useful from the standpoint of increasing the size of mounted batteries to thereby improve energy density per unit weight or improve output energy density or for attaining life prolongation of batteries.
However, LiFePO4 is known to be lower in the electronic conductivity of inner parts of the positive-electrode active material and in high-rate discharge characteristics as compared with LiCoO2, LiNiO2, LiMnO2, Li(Ni1/3Mn1/3CO1/3)O2, and the like.
Furthermore, nonaqueous-electrolyte batteries employing LiFePO4 as a positive-electrode active material have had the following problem. When these batteries are repeatedly charged and discharged in a high-temperature environment of, for example, about 60° C., elements including iron which are contained in the active material partly dissolve away with repetitions of charge/discharge, and the dissolved iron adversely affects the negative-electrode active material constituted of a carbon material, etc. As a result, the negative electrode itself is impaired in charge/discharge reversibility and other properties and is hence reduced in reactivity, and this tends to result in a decrease in the capacity or output of the nonaqueous-electrolyte batteries.
Patent document 1 discloses a nonaqueous-electrolyte battery which includes a positive-electrode mix layer that includes a positive-electrode active material including iron lithium phosphate, a conductive material, and a binder and has a density regulated to 1.7 g/cc and which further includes a nonaqueous electrolytic solution containing a solvent including ethylene carbonate and a chain ether, as a nonaqueous-electrolyte battery which can have an improved discharge capacity even during high-rate discharge in which the battery is discharged at a relatively high current.
Patent document 2 discloses a nonaqueous-electrolyte battery which includes a positive-electrode active material including iron lithium phosphate of an olivine structure as a main component, an electrolyte including LiPF6 as a main component, and a nonaqueous solvent that includes, as a main component, a mixed solvent composed of ethylene carbonate and diethyl carbonate or a mixed solvent composed of ethylene carbonate and ethyl methyl carbonate and that further contains at least vinylene carbonate and/or vinylethylene carbonate, as a nonaqueous-electrolyte battery which has a high capacity and high output and can retain the high capacity equal to the initial value even after repeatedly charged/discharged in a high-temperature environment of, for example, 60° C., and which is prevented from decreasing in ordinary-temperature output and low-temperature output, for example, output at around −30° C., and shows the high output equal to the initial value.